1. General Structure of Driving Stability Control (DSC)
The term driving stability control (DSC) covers four principles of influencing the driving behavior of a vehicle by means of predeterminable pressures in individual wheel brakes and by interfering with the engine management of the driving engine. These include brake slip control (ABS) which is to prevent locking of individual wheels during a brake operation, traction slip control (TCS), which prevents the spinning of the driven wheels; the electronic brake force distribution system (EBD), which controls the ratio of the brake forces between the front and the rear axle of the vehicle; and a yaw torque control system (YTC), which ensures stable driving conditions during travel in a curve.
Consequently, a vehicle is defined in this connection as a motor vehicle with four wheels, which is equipped with a hydraulic brake system. In a hydraulic brake system, brake pressure can be built up by the driver by means of a pedal-actuated master cylinder. Each wheel has a brake, with which one inlet valve and one outlet valve each is associated. The wheel brakes communicate with the master cylinder by way of the inlet valves, while the outlet valves lead to a pressureless tank or to a low-pressure accumulator. Finally, there also is an auxiliary pressure source, which is able to build up a pressure in the wheel brakes regardless of the position of the brake pedal. The inlet and outlet valves can be electromagnetically actuated for pressure regulation in the wheel brakes.
To detect states in the dynamics of the vehicle movement, there are four speed sensors, one per wheel, one yaw rate meter, one lateral accelerometer, and at least one pressure sensor for the brake pressure generated by the brake pedal. The pressure sensor may be replaced with a pedal travel or pedal force meter if the auxiliary pressure source is arranged such that a brake pressure built up by the driver is not distinguishable from that of the auxiliary pressure source. In addition, it is possible to poll information about the condition of the transmission, e.g. about gearshift control, etc.
A fall-back solution is advantageously put into practice in light of such a large number of sensors. This means that, in the case of failure of part of the sensor system, only the component of the control system that depends on that part is switched off. If, for example, the yaw rate meter fails, no yaw torque control can be performed, but the ABS, TCS and EBD systems continue to function. The driving stability control can consequently be limited to these other three functions.
In a driving stability control, the driving behavior of a vehicle is influenced such that the driver will be better able to master the vehicle in critical situations, or critical situations will be avoided to begin with. A critical situation is defined herein as an unstable driving condition in which, in the extreme case, the vehicle does not follow the driver's instructions. The function of the driving stability control is consequently to impart to the vehicle the behavior desired by the driver in such situations within the physical limits.
While the longitudinal slip of the tires on the road surface is mainly of significance for the brake slip control system, the traction slip control system and the electronic brake force distribution system, the yaw torque control system (YTC) also involves additional variables, e.g., the yaw rate {dot over (Ψ)}.
Various vehicle reference models may be used for yaw torque control. The calculation is simplest on the basis of a single-track model, i.e., the front wheels and the rear wheels are integrated in this model into one wheel each, which is located on the longitudinal axis of the vehicle. The calculations become considerably more complicated if they are based on a two-track model. However, since lateral displacements of the center of gravity (rolling movements) can also be taken into account in the two-track model, the results are more accurate.
The system equations
                              β          .                =                                            c              11                        ⁢                          β              v                                -                      Ψ            .                    +                                    c              12                        ⁢                                          Ψ                .                                            v                2                                              +                                    c              13                        ⁢                          δ              v                                                          F        ⁢                                  ⁢        1.1                                          Ψ          ¨                =                                            c              21                        ⁢            β                    +                                    c              22                        ⁢                                          Ψ                .                            v                                +                                    c              23                        ⁢            δ                                              F        ⁢                                  ⁢        1.2            can be written in the phase space diagram for a single-track model.
The sideslip angle β and the yaw rate {dot over (Ψ)} represent the phase variables of the system. The input variable acting on the vehicle is the steering angle δ, as a result of which the vehicle receives the yaw rate {dot over (Ψ)} as an output variable. The model coefficients cii are formed as follows:
                                                                        c                11                            =                              -                                                                            c                      h                                        +                                          c                      v                                                        m                                                                                                        c                12                            =                                                                                          c                      h                                        ⁢                                          l                      h                                                        -                                                            c                      v                                        ⁢                                          l                      v                                                                      m                                                                                        c                13                            =                                                c                  v                                m                                                                                                        c                21                            =                                                                                          c                      h                                        ⁢                                          l                      h                                                        -                                                            c                      v                                        ⁢                                          l                      v                                                                      Θ                                                                                        c                22                            =                              -                                                                                                    c                        h                                            ⁢                                              l                        h                        2                                                              +                                                                  c                        v                                            ⁢                                              l                        v                        2                                                                              Θ                                                                                                        c                23                            =                                                                    c                    v                                    ⁢                                      l                    v                                                  Θ                                                                        F        ⁢                                  ⁢        1.3            ch and cv are the resulting rigidities from the elasticity of the tire, wheel suspension and steering on the rear axle and the front axle, respectively. lh and lv are the distances of the rear axle and the front axle, respectively, from the center of gravity of the vehicle. Θ is the moment of inertia of the vehicle, i.e., the moment of inertia of the vehicle around its vertical axis.
Longitudinal forces and displacements of the center of gravity are taken into account in this model. This approximation is also valid only for low angular velocities. Consequently, the accuracy of this model decreases with decreasing curve radii and increasing velocities. However, the amount of calculations is manageable. Further explanations of this single-track model can be found in the book Fahrwerktechnik: Fahrverhalten [Chassis Engineering: Driving Behavior] by Adam Zomotor, Vogel Buchverlag, Wurzburg, 1987.
A two-track model, whose accuracy is superior to that of a single-track model, is proposed for a vehicle in DE-40 30 704 A1. The yaw rate {dot over (Ψ)} and the sideslip angle β form the phase variables in this case as well. However, when a two-track model is used, it is necessary to consider the fact that an enormous calculation capacity is needed to make it possible to perform a control intervention in a relatively short time.
The methods and the control systems are used to create an additional torque by targeted interventions at the individual brakes of a vehicle, which torque leads by way of the actually measured yaw variation per time unit (actual yaw rate) of a vehicle to the yaw variation per time unit (desired yaw rate) which is influenced by the driver. Hence, a method and a control system will particularly intervene in a supporting manner into the steering performance of the vehicle when due to certain conditions (e.g. excessive speed, slippery roadway) the curved track actually covered by the vehicle does not correspond to the curved track desired by the driver without additional torque. In principle, methods and control systems of this type to improve driving stability have already been described comprehensively and, therefore, shall not be explained again in detail. In methods and control systems of this type, input parameters which result from the curved track desired by the driver (e.g. steering wheel angle, driving speed) are always sent to a vehicle model circuit which, on the basis of a known single-track model or another driving model, determines a nominal yaw rate (ψDesired) from these input parameters and from parameters being characteristic of the driving performance of the vehicle, yet also from quantities (coefficient of friction of the roadway) predetermined by the characteristics of the ambience, which yaw rate is compared to the measured actual yaw rate (ψactual). The difference of the yaw angles (ΔψDiff) is converted by means of a yaw torque controller into an additional yaw torque MG which forms the input parameter of a distribution logic unit.
The distribution logic unit itself defines the brake pressure to be applied to the individual brakes, optionally in dependence on the braking request of the driver demanding a defined brake pressure at the wheel brakes. This brake pressure shall produce an additional torque at the vehicle, in addition to the optionally desired brake effect, which supports the driving behavior of the vehicle in the direction of complying with the steering request of the driver.
If due to external conditions during driving or conditions being due to the performance of the driver, variations of the vehicle-dynamic driving performance (e.g. changes in the coefficient of friction) will occur, e.g. a change in the engine torque, e.g. due to release of the accelerator or push-down of the accelerator, or due to braking, the driving behavior of the vehicle will change because, among others, there will be a change of the axle load and, thus, of forces which is induced by the interaction of several influences such as tire influences, kinematical influences and elasto-kinematical influences.
For example, when driving through a curve and release of the accelerator occurs, the driving forces Fa are active at the drive wheels prior to the release of the accelerator. Due to the lateral deformation of the tire contact area, the longitudinal driving force FA=2×Fa will act in dependence on the lateral forces slightly outside the wheel center plane. An understeering yaw torque ({dot over (Ψ)}understeer) is produced due to the longitudinal driving force FA that acts asymmetrically in relation to the vehicle longitudinal axis.
After the release of the accelerator, the engine (and other resistances) slows down the vehicle, the (longitudinal) driving forces become negative. In addition, the deceleration produces an inertia force mx in the point of gravity SP, with the result that the axle load increases at the front wheels and decreases by the same amount at the rear wheels. Thus, the distribution of the transmittable lateral forces changes. The lateral force change (lateral force on the front axle rises slightly and decreases greatly at the rear axle) produces an oversteering yaw torque ({dot over (Ψ)}oversteer), the slip angles at the rear axle will increase and the vehicle turns into the curved track. When the engine torque is changed from driving force into brake force, the reversal of theses torques will induce a change of the driving behavior of the vehicle from an oversteering to an understeering driving behavior.
There are methods fulfilling only partial aspects of the mentioned requirement.
Thus, the function ‘ABS-plus’ is known for partial brake operations in the curve. This function achieves stabilization of the vehicle by pressure reduction on the inside wheels in a turn. However, ABS-plus detects the vehicle performance exclusively from the measured wheel speeds.
When the driver slows down in a curve to such an extent that ABS control is triggered, already the ABS function itself is often capable of counteracting the tendency of turning into a bend. The reason for this is that a greater vertical force and, thus, a higher potential of longitudinal force prevails on the exterior curve side than on the interior curve side. ABS safeguards the optimum utilization of the longitudinal force potential. The resulting unbalance of forces will then bring about a stabilizing yaw torque.
It is disadvantageous that these methods do not have a controller of their own but share the yaw torque controllers with the standard ESP. Their effect evolves because they take influence on the parameters of the YTC controllers (e.g. decrease of the control thresholds).
The previous methods suffer from the following disadvantages:                1. Each method acts only in one or in a few defined driving situations and is limited to one defined intervention strategy only.        2. Each method includes non-optimal partial solutions; e.g. the comfort is optimal in one method because the hydraulic pump provides its full output for pressure build-up. In the other method, the reference yaw rate is not used continuously.        3. To cover a rising number of driving situations by a simultaneous activation of an increasing rate of single methods will quickly reach limits because the ranges of influence (driving situations) of the methods can overlap each other what is not desired, or they may leave gaps, and the intervention strategies contradict each other in part.        
In view of the above, an object of the invention is to provide a method and a control system for improving the driving behavior of a vehicle which influences the driving behavior of a vehicle in such a manner that it is adapted early and comfortably to the desired driving behavior depending on the driving situation. Another objective is to enhance the sensitivity of response of the control.